This invention relates to novel plastic compositions having enhanced environmental degradability.
The advent of plastics has given rise to improved methods of packaging goods. For example, polyethylene and polypropylene plastic films, bags, bottles, styrofoam cups and blister packages have the advantages of being chemically resistant, relatively unbreakable, light in weight and translucent or transparent. The increasing use of plastics in packaging applications has created a serious waste disposal problem. Burning of these plastic materials is unsatisfactory since it adds to air pollution problems.
Unlike some other packaging materials, such as paper and cardboard, plastics are not readily destroyed by the elements of nature. Thus, burying them is not an effective means of disposal, and can be expensive.
Plastics are biologically recent developments, and hence are not easily degradable by micro-organisms which attack most other forms of organic matter and return them to the biological life cycle. It has been estimated that it may take millions of years for organisms to evolve which are capable of performing this function. In the meantime, plastic containers and packaging films are beginning to litter the countryside after being discarded by careless individuals.
One approach to the alleviation of the problem of plastics waste and litter would involve the development of novel polymeric compositions which undergo accelerated degradation under environmental conditions.
In preparing photosensitive polymeric compositions for use in fabricating environmentally degradable plastic products, the usual approach is to incorporate a single photosensitizing additive. Within limits, the degradability of the compositon can be regulated by varying the concentration of the sensitizing additive. This approach is satisfactory in a number of applications, e.g., where the product is a thin film of a polymer which undergoes relatively facile photodegradation.
In attempting to prepare photodegradable plastic articles of reasonable thickness, such as plastic bottles having a wall thickness of 0.01-0.05 inch, the aforementioned "prior art" method is not always effective; this is particularly true when the polymer is relatively resistant to photo-oxidative degradation. The relatively slow degradation rates observed in the case of thick-walled samples may be attributed to the following factors.
Firstly, though sunlight is a broad-spectrum source of ultraviolet and visible radiation, only those wavelengths of light corresponding to the ultraviolet absorption band of the photosensitizing additive are actually effective in promoting photodegradation; light of other wavelengths is not effective in promoting photodegradation, since it is not absorbed, and hence energy from light of these wavelengths is "wasted".
Secondly, there is a limit to the extent to which the rate of photodegradation can be increased by increasing the concentration of a single photosensitizing additive. If the concentration is too high, essentially all of the light of photochemically active wavelengths is absorbed near the exposed surface of the sample, and the remainder of the material is actually protected from photodegradation.
Such difficulties can be overcome, at least in part, by the use of degradable compositions containing two or more photosensitizing additives, particularly if the sensitizers absorb radiation in different regions of the ultraviolet spectrum. This approach makes it possible to maximize the energy absorbed from sunlight and hence, if the additives are photochemically active, to maximize the rate of photodegradation.
The use of multiple-sensitizer additive systems also offers an approach to the problem of excessive absorption of UV radiation near the surface of a thick-walled article, since lower concentrations of the individual sensitizers can be used, and since degradation will then involve light of two or more photochemically active wavelength ranges, each with its own characteristic relation between intensity and distance from the surface of the article. By the proper choice of sensitizing additives and adjustment of their concentrations, compositions of substantially improved degradability can be obtained. In some cases, it may be desirable to employ a high concentration of one additive, which absorbs strongly in a given wavelength range, in order to promote rapid photodegradation near the surface, while simultaneously employing a lower concentration of a second additive which absorbs in a different wavelength range, in order to promote more gradual degradation of the remainder of the material. The first sensitizer will then promote surface embrittlement and cracking, while the second will promote subsequent disintegration of the bulk of the material, e.g., by facilitating the propagation of cracks initiated at the surface.